Carbonaceous materials such as low rank coals and biomass, e.g., peat and cattails, may be used as a substitute source of energy for high grade coals. They often contain less sulfur than many of the deposits of high rank coals and are generally more readily obtainable. These materials, however, generally contain more water and provide less energy (per unit weight) than do high rank coals.
Low rank or low grade carbonaceous materials or coals include lignite, brown coal, and subbituminous coal. Such carbonaceous low grade coals are characterized in that they have not undergone sufficient geological metamorphosis to be converted into high grade hard coals such as bituminous or anthracite.
Coal is a naturally occurring solid material that is an aggregate of undifferentiated hydrocarbonaceous solids (at room temperature), oxygen, water, and a wide assortment of inorganic minerals. Because the properties of coal vary widely between each naturally occurring deposit, attempts have been made to define broad classifications which group coal into a more coherent structure. The four most broadly used classes of coal, in descending quality, or rank, are: anthracite, bituminous, subbituminous, and lignite. In this context, "quality" can be interpreted to mean an increasing proportion of hydrocarbonaceous, i.e., carbonaceous, material per unit amount of coal. Conversely, increasing quality can also mean a decreasing proportion of the detritus, e.g., oxygen, water, and mineral content in the coal. Because there is a great diversity between the properties of each individual coal, the broad classes of coal are often subclassified as need requires: for example, Subbituminous High Volatile A, B, or C. Generally though, if a coal is to be used as a source of combustion energy, the larger the proportion of hydrocarbons per unit of coal, the better a fuel it will be.
Because the size of deposits of coal are frequently on a geologic scale, they are often referred to by their location. Another technique is to additionally refer to a coal in its relation to other sub-deposits, or seams, within the overall deposit. A common example frequently mentioned in coal literature is Illinois Bituminous Number 6. The information contained within this identity is that the coal is a bituminous coal, it is located in Illinois (USA), and it is from the sixth seam. This specifies a very particular type of coal, with relatively well documented properties. Apparently, the reason for this nomenclature system lies in the ultimately singular properties of each coal deposit.
Generally, the wider the geographic area described, the more general, or broadly, the characteristics of the coal type would be interpreted. For example, a relatively large coal deposit stretches north across the states of Wyoming and Montana. This is often referred to as the Powder River Basin deposit. Although it is understood that each specific deposit within the area will have its own individual characteristics, a Powder River Basin coal generally describes a subbituminous coal with relatively low sulfur content and relatively high moisture content. Referring to a specific coal seam within the Powder River Basin, such as Rosebud, defines a coal with the general characteristics of a Powder River Basin coal, but differing somewhat in specifics. In this case, Rosebud coal has a somewhat higher sulfur content and a mineral content specific to the Rosebud seam within the Powder River Basin.
Another similar example of common coal nomenclature can be found in North American lignites. Large deposits of lignitic coal occur in both North Dakota and Texas. As expected, these are often referred to as North Dakota lignite and Texas lignite, to specify their general properties from the even broader classification of being a lignitic coal. Referring to a specific deposit within these major deposits, such as a Center, North Dakota lignite, defines the coal even further.
There are classification systems that provide a further characterization of these materials in various ways. For example, carbonaceous materials may be classified according to their heating value. High rank coals are generally considered to be those coals that possess a heating value of greater than about 10,000 BTUs/pound, whereas the heating value of low rank coals is generally less than this value. For example, the heating value, or the "power" per unit weight, of bituminous coals is around 11,000 BTUs/pound and that of lignite is around 7,000 BTUs/pound.
Low rank coals often contain about 30% to 70% by weight moisture (H.sub.2 O ) as mined. They also often contain up to about 20% by weight oxygen, excluding the amount contained in the H.sub.2 O. This oxygen is generally contained in nondecomposed organic detritus, and is typically in the form of carboxyl (--COOH) groups present in residual acids, such as humic acids, as well as sodium carboxylate (--COONa) and perhaps other metal carboxylates (--COOM), that may be present. That is, the oxygen content found in most low rank coals and peat (exclusive of that within the 30% to 70% moisture) results from the partial decomposition of products of organic matter. Biomass often contains an even higher moisture and oxygen content. Both water and oxygen are generally undesirable, at least in part because they represent weighty impurities. Furthermore, the oxygen content present in the --COOH and --COONa groups can make the material relatively reactive, which is not always desirable.
Because deposits of low rank coals and biomass are relatively vast, are sometimes less expensive to mine, and generally have relatively low sulfur contents, there is a need to render these materials more useful and efficient as fuels. This can be done, in part, by reducing the moisture and oxygen content of the mined or collected material (raw). A reduction in the sodium content also tends to improve certain handling characteristics of the material. It is also important to reduce the moisture and oxygen content of these materials in order to reduce the costs associated with transportation. For example, the relatively weighty impurities can readily result in the expenditure of millions of extra dollars in transportation costs.
Although some of the moisture content is present as surface water, which can generally be readily removed, a large portion of the water is inherent water. That is, either through chemical or physical forces the water is bound or trapped within the carbonaceous material in a manner such that it cannot be readily removed by simple physical manipulations such as filtering, decanting, or draining.
There are known methods for removal of the surface and inherent water from carbonaceous materials, such as low rank coals and biomass. Among them are processes that involve air drying. Processes known for partial or complete drying of the moist lignitic-type coal by air drying, however, generally result in a dried coal that readily crumbles or disintegrates into fine-sized particles and dust. This fine dust generally poses problems in storage and during shipment, at least in part due to their propensity for spontaneous combustion. Furthermore, the drying processes are inefficient because they generally result in consumption of large amounts of energy due to the evaporation process involved.
There are also methods known that utilize steam treatment and hot water treatment for the removal of inherent water from these materials. It is known that at temperatures generally in the range of 230.degree. C. to 330.degree. C., application of these methods to coal generally causes permanent loss of chemically and/or physically bound or trapped inherent water. Further, the surface of the coal is modified to a more hydrophobic state that reduces the tendency for water to adhere. Surface moisture associated with pores or capillaries is generally reduced as well. Furthermore, application of pressure exceeding the vapor pressure of water at elevated temperatures (during these processes) inhibits evaporation of the freed water. This generally is advantageous because the costs of dewatering, due to the extensive energy requirements for evaporation processes, are reduced.
Some of the known hot water or steam treatment processes involve the use of batch autoclaves at elevated temperatures with pressures greater than the vapor pressure of water at such temperatures. Other systems use rotary preheating and processing kilns.
There are also systems that use a pressurized reactor which allow for extraction of undesired inherent water and excess oxygen from low rank carbonaceous solids by the countercurrent flow of a processing liquid, e.g., water, and the solids. See, for example, P. B. Tarman et al. in U.S. Pat. No. 4,579,562. Generally, this system is used for lump size coal, that is, coal with a particle size of greater than about 1/4 inch (6 mm).
Hydraulic lockhopper arrangements are known and used for both feeding coal into, and discharging beneficiated coal from, such systems. These allow for the transfer of the material between ambient pressure and the reactor pressure. In certain reactor systems the feeding and discharging systems also incorporate means for draining or dewatering the coal.
Generally, for the dewatering of relatively fine particle size coal or biomass, pumpable slurry systems are used. The relatively fine solid particles, i.e., with a particle size of less than about 1/4 inch (6 mm), are entrained in a water-based slurry, which can be pressurized. The slurry is then passed through a heat exchanger for treatment. In certain systems pressurized oxygen or oxygen-containing gases are introduced to enhance the oxidation of organic matter therein. This serves as a source of heat for the dewatering process.
There are also systems known that use chemical additives during the thermal treatment of carbonaceous materials. For example, nonaqueous volatile solvents have been added to moist particulate carbonaceous material to displace the water. The addition of organic and inorganic acids has been shown to improve the process of reducing the inherent water content of coal. Other systems dry high moisture content carbonaceous solids by treating the material with hydrocarbons such as naphtha, fuel oil, low grade petroleum fractions, tar, or the like. This is done for a variety of reasons such as, for example, agglomeration of solid particles under turbulent conditions.
Certain of these systems are limited in the minimum particle size and density of the material that can be processed. Very small particles often create problems associated with, for example, plugging of certain systems. Also, materials close in density to that of water limit certain beneficiating systems to the use of less dense organic liquids, particularly in countercurrent extraction systems.
Further, certain of the beneficiating systems, and particularly the countercurrent extraction systems, use extraneous processing steps and large volumes of water that reduce the efficiency of the process and increase the costs associated with the processing. It would be generally desirable to reduce the amount of water that is needed for conservation purposes, to reduce the amount of waste water that is treated, and to more efficiently utilize energy resources.
What is needed is a more efficient beneficiation method that improves the fuel properties of a wide variety of carbonaceous solids, such as low rank coals and biomass. What is particularly needed is a system which is well adapted to handle not only solids comprising large particles, but also those solids containing materials of relatively low density and relatively small particle size. Specifically, a method is needed that more efficiently reduces the moisture, oxygen, and sodium content of carbonaceous materials, thereby upgrading the heating value of the material while improving the mechanical handling and combustion properties of the solids. Such improvements could at least in part lie in an achieved reduction in the amount of resources required to operate the process. These resources may include, but are not limited to, water, energy, and financial resources.